1. Field of the Invention
The present invention relates generally to optical pickup devices and more particularly to an optical pickup device in which a light emitting element and a light receiving element are formed integrally on a semiconductor substrate.
2. Description of the Prior Art
Optical heads with various structures have been proposed so far to detect an information from information pits formed on the surface of a compact disc (CD) or the like. A perspective view forming FIG. 1 illustrates a structure of an optical pickup device employing a composite light emitting and/or light receiving element that we have previously proposed as Japanese Patent Application No. 61-38575.
As FIG. 1 shows, there is provided an optical head 1 in which a first photo detector 3 made of a so-called PIN diode or the like is formed, for example, on the left-hand side half of a major surface 2A of a rectangular semiconductor substrate 2 made of silicon or the like as a light receiving element. The first photo detector 3 is formed of, for example, two sets of photo detectors 3a and 3b, each of which is divided to provide three photo detecting portions. A second photo detector 4 made of a PIN diode or the like for monitoring is formed on the right-hand side half of the major surface 2A of the semiconductor substrate 2 as a light receiving element, if necessary. Between the first and second photo detectors 3 and 4, a light emitting element 5 such as a semiconductor laser chip or the like is directly soldered to the major surface 2A of the semiconductor substrate 2. Also, there is provided an optical path branching part, i.e., a prism 6. This prism 6 is trapezoidal in cross section and is mounted on the major surface 2A of the semiconductor substrate 2 to cover the first photo detector 3, thus a composite light emitting/light receiving element 1a being formed. The prism 6 has a face 6a opposing to the light emitting point of the active layer in the semiconductor laser chip 5. This face 6a is formed as a semitransparent reflection face. Meanwhile, in a face 6b of the prism 6 contacting with the semiconductor substrate 2, other face than the face contacting with the photo detectors 3a and 3b and a face 6c of the prism 6 opposing the face 6b are each formed as a reflection face.
With the above-mentioned arrangement, a laser beam 7a emitted from the active layer of the semiconductor laser chip 5 is reflected on the semitransparent reflection face 6a of the prism 6 and is then irradiated on an optical disc through an objective lens (not shown) as an incident laser beam 7b. The light of the incident laser beam 7b reflected on the optical disc travels through the face 6a of the prism 6 and becomes incident on the first set of photo detector 3a. The light passed through the face 6a is reflected by a semitransparent layer (not shown) formed between the prism 6 and the first set of photo detector 3a and is then reflected on the face 6c of the prism 6 to become incident on the second set of photo detector 3b, whereby the data corresponding to the information pits on the optical disc are detected. A laser beam 7c is emitted from an opposite active layer of the semiconductor laser chip 5 for monitoring.
In the abovementioned composite light emitting/light receiving element 1a, a part of the laser beam, which is emitted from the semiconductor laser chip 5, travels through the prism 6 and directly becomes incident on the first photo detector 3 and it, becomes an undesired light (or stray light) which brings about a noise to the detecting signal. Therefore, We have previously proposed various means for eliminating the aforesaid undesired light.
From the experimental results, it was noted that while the undesired light incident on the prism 6 is removed in the conventional composite light emitting/light receiving element 1a, other kind of undesired light is produced to exert a bad influence upon the detecting signal that the first photo detector 3 produces.
Referring to FIG. 2, let us now explain why such other kind of undesired light is produced. FIG. 2 is a fragmentary cross-sectional side view taken through the line II--II of FIG. 1, looking in the direction represented by an arrow A. Except for the semitransparent reflection face 6a of the prism 6 formed on the major surface 2A of the semiconductor substrate 2 to cover the first photo detector 3, left, right and rear side wall faces 6d, 6e and 6f are formed as such faces that are not calendered. For example, they are formed as smoked-glass faces. For this reason, a part of the laser beam 7a emitted from the active layer of the semiconductor laser chip 5 travels through the semitransparent reflection face 6a and is then reflected and/or scattered on the side wall faces 6d, 6e and 6f in the prism 6. Thus, as shown by reflected and/or scattered laser beams 8 represented as broken lines in FIG. 2, an undesired light again becomes incident on the light receiving faces of the photo detectors 3a and 3b with a small incident angle .theta.1. Such an undesired light is superimposed upon an RF signal from the first photo detector 3 as a DC offset signal, thereby to exert a bad influence on the detecting signal. Other reason that there exist much undesired light beams which are repeatedly reflected and/or scattered within the prism 6 is that the refractive index of the material forming the prism 6 is larger than the refractive index of the air and hence the undesired light beams are not radiated from the prism 6 to the air.